Control of an olyte-catholyte concentrations in membrane cells

ABSTRACT

The electrolysis of aqueous sodium chloride in an electrolytic cell divided into anolyte and catholyte compartments by a hydraulically impervious cation-permselective membrane is improved, especially with respect to current efficiency, by operation at a sodium hydroxide concentration within the catholyte in the range of 31-43 percent, controlled by the maintenance of an average sodium chloride concentration in the anolyte within the range of 120-250 grams per liter, the only source of water to the catholyte compartment being that transported through the membrane.

United States Patent 5 Stacey et a].

[ 51 Nov. 20, 1973 3,312,609 4/1967 Kircher 204/98 3,547,791 12/1970 Mellish et al. 3,654,104 4/1972 Yoshida et Primary Examiner-John H. Mack Assistant ExaminerA. C. Prescott AttorneyRoy Davis et a1.

[57 ABSTRACT The electrolysis of aqueous sodium chloride in an electrolytic cell divided into anolyte and catholyte compartments by a hydraulically impervious cationpermselective membrane is improved, especially with respect to current efficiency, by operation at a sodium hydroxide concentration within the catholyte in the range of 31-43 percent, controlled by the maintenance of an average sodium chloride concentration in the anolyte within the range of 120-250 grams per liter, the only source of water to the catholyte compartment being that transported through the membrane.

2 Claims, 1 Drawing Figure [75] Inventors: Alan J. Stacey, Garfield Heights;

Ronald L. Dotson, Mentor, both of Ohio [73] Assignee: Diamond Shamrock Corporation,

Cleveland, Ohio [22] Filed: Mar. 9, 1972 [211 App]. No.: 233,129

[52] US. Cl 204/98, 204/180 P [51] Bold 13/02, COld l/06 [58] Field of Search 204/98, 95, 94, 128, 204/180 P [56] References Cited UNITED STATES PATENTS 2,827,426 3/1958 Bodamer 204/98 3,220,941 11/1965 Osborne 204/98 X Z LU E 70 h l l l I I l l I PAIENTEDrmvzo ms /o CURRENT EFFICIENCY l l l I NuOH BACKGROUND OF THE INVENTION A large portion of the chlorine and caustic produced throughout the world is manufactured in diaphragmtype electrolytic cells wherein the opposed anode and cathode are separated by a fluid permeable diaphragm, usually of asbestos, defining separate anolyte and catholyte compartments. In. operation, saturated brine is fed to the anolyte compartment wherein chlorine is generated at the anode, the brine percolating through the diaphragm into the catholyte compartment wherein sodium hydroxide is produced in a concentration within the range of l l-l8 percent and contaminated." with large amounts of sodium chloride. This sodium chloride must then be separated from the caustic and the caustic in turn concentrated by evaporation toprovide a commercial product.

Through the years, substitution of a membrane material for the diaphragm has been proposed. These membranes are substantially impervious to hydraulic flow. In operation, brine is again introduced into the anolyte compartment wherein chlorine is liberated. Then, in the case of a cation permselective membrane, sodium ions are transported across the membrane into the catholyte compartment. The concentration of the relatively pure caustic produced in the catholyte compartment is determined by the amount of water added-to this compartment from a source exterior the cell. While operation of a membrane cell has many theoreticaladvantages, its commercial application to the production of chlorine and caustic has been hinderedowing-to the often erratic operating characteristics of the cells.

STATEMENT OF THE INVENTION Therefore it is an object of the present invention to provide an improvement in the operation of membrane-type electrolytic cells for the production ofchlorine and caustic.

It is a further object of the present invention to provide an improvement in the operation of such membrane cells whereby stability of operation is achieved at a maximum current efficiency. 7

These and further objects of the present invention will become apparent to those skilled in the art from the specification and claims which follow.

An improvement has now been found in a process for the electrolysis of an aqueous sodium chloride solution in an electrolytic cell, the anolyte and catholyte compartments of which are separated by an electrolytically conductive, hydraulically impervious cationpermselective membrane, which improvement consists essentially of establishing an initial sodium hydroxide concentration in the catholyte in the range of 31-43 percent, and maintaining said sodiumhydroxide concentration within said range by controlling the average sodium chloride concentration of the anolyte within the range of 120-250 grams per liter, the sole source of water to the catholyte being that transported through said membrane. The stated sodium hydroxide concentration appears critical to the operation of the cell at an optimum current efficiency. Moreover, control of the sodium chloride concentration in the anolyte within the stated range results in maintaining the caustic concentration in the catholyte at an optimum value without necessity for the separate controlled addition of water to the catholyte. Thus a self-sustaining equilibrium is established and maintained between the anolyte and the catholyte resulting in consistent and efficient cell operation.

DESCRIPTION OF THE DRAWING The attached FIGURE is a graph relating sodium hydroxide concentration to current efficiency and showing a critical optimum value within the range of the present invention, the particular values being those determined under the operating parameters obtaining in the second production run of the Example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The membrane cells to which the operational techniques of the present invention apply, as well as the other operating parameters, are for the most part conventional. Generally, an enclosure is provided and dividedinto two compartments by the membrane material. In one compartment, the catholyte compartment, is disposed an appropriate cathode, generally a metallic material, such as iron. The other compartment, the anolyte compartment, contains the anode a conductive, electrolytically-active material, such as graphite or, more desirably, a dimensionally stable anode, e.g., a titanium substrate bearing a coating of a precious metal; precious metal oxide or other electrolyticallyactive corrosion-resistant material. The anolyte compartment is provided'with an outlet for generated chlorine gas, an inlet for brine solution and an outlet for depleted brine. Similarly, the catholyte compartment will have outlets for liquid and gaseous products and, generally, an inlet through which water and/or sodium hydroxide solution may be added initially.

In operation, a direct currrnt is passed between the electrodes, causing the generation of chlorine at the anode andthe selective transport of hydrated sodium ions across the membrane into the catholyte compartment where they combine with hydroxide ions formed at the cathode by the electrolysis of water, hydrogen gas being liberated.

In general a cell employing any permselective cationexchange membrane electrolytically conductive in the hydrated'state obtaining under cell conditions and useful in the electrolysis of brine is improved by the operational-techniques characteristic of the present invention. Most often these membranes are sulfonated materials based upon a chemically-resistant highly crosslinked polymer backbone, such as a divinylbenzeneacrylic acid copolymer, polyethylene, divinylbenzenepolystyrene copolymers, polyvinylfluorocarbon ethers and the like. Particularly preferred at this time, because oftheirapparentsuperiority in a membrane cell for the electrolysis of sodium chloride, are the membranes manufactured and sold by the E. l. duPont deNemours & Co. under the trademark of XR perfluorosulfonic acid-membranes and based upon a completely fluorinatedvinyl ether polymer containing pendant sulfonic acid groups and having an equivalent weight within the range of l-,0O0-2,200 (grams of polymer per equivalent of proton) and an average gel water content within the range of 15-40 percent. Thus, generally, a useful membrane comprises a chemically and mechanically resistant polymer matrix or backbone to which are attached, in an extremelyv stable fashion, highly-electronegative exchange sites, such as sulfonic, phosphonic or carboxylic acid groups. The desired high degree of electrolytic conductivity and high apparent sodium ion transport number are contingent upon the presence in these membranes of a considerable quantity, generally in excess of Weight percent on a dry resin basis,of

gel water. A further understanding of these membranes and examples thereof may be found by reference to U. S. Pat. Nos. 2,636,851; 3,017,338; 3,496,077; 3,560,568; 2,967,807; 3,282,875 and British Patent 1,184,321.

The thicknesses of membranes of the foregoing type employed are generally on the order of 4-20 mils. Greater thicknesses are useful, however, any incidential advantage in the use of such thicknesses is more than offset by the added cost of the material. With thicknesses of less than 10 mils, mechanical support, e.g., in the nature ofa woven Teflon screen, is often advantageous.

Exemplary of other non-critical process parameters are operating temperatures within the range of l00C., feed brine pH within the range of 1.0-6.0 and anode current densities on the order of 1.0-5.0 amperes per square inch.

Briefly stated, the invention contemplates that once there is established a caustic concentration corresponding to the optimum sodium hydroxide-based current efficiency obtainable with the permselective membrane, this concentration is maintained by control of the anolyte. Such control ensures that the proper critical amount of hydrated sodium ions will be transported by the membrane to the catholyte, said transport being the only source of water to the catholyte.

As the attached FIGURE discloses, the range of sodium hydroxide concentrations in the catholyte which corresponds to optimum current efficiency is quite narrow, generally within 31-43 percent by weight, especially 35-39 percent, and peaking in the area of 36-38 percent. At lesser concentrations, a much lower current efficiency is realized and, in any event, more water must be evaporated from the catholyte to yield a commercial product. At higher concentrations, current efficiency again sharply decreases, cell voltage increases and the catholyte product rapidly becomes too viscous to handle, often setting-up into a hard mass at concentrations greater than 55 percent.

The means of maintaining the caustic concentration at the desired level is to be found, according to the invention, in the concentration of sodium chloride present in the anolyte compartment. This concentration is considerably lower than heretofore employed for optimum diaphragm or membrane cell operation and is generally within the range of 120-250, especially 150-220, grams per liter. As discussed hereinbelow, the concentration of sodium chloride in the anolyte is not to be confused with the feed brine concentration since other factors, e.g., depletion and flow rate, must be taken into account. The concentration referred to is that within the anolyte compartment, often conveniently measured by determining the salt content of the effluent from this compartment. At concentrations lower than l grams per liter, insufficient sodium ions are presented to the membrane for transport, oxygen content of the chlorine gas increases, the brine is generally less conductive and caustic concentration decreases. On the other hand, and unexpectedly, if average concentrations in excess of 250 grams per liter are found in the anolyte compartment, the caustic concentration becomes unstable, that is, a continuous increase in caustic concentration is noted without any apparent leveling off at some higher, less efficient, concentration.

The means of initially establishing the sodium hydroxide concentration within the desired range, particularly at about 36-38 percent, are quite varied. Conveniently, and to avoid delays in reaching equilibrium conditions, the catholyte compartment may be initially charged with a caustic solution of the desired concentration. Thereafter, upon imposition of an electrolyzing current and establishment of the appropriate brine concentration, the caustic concentration will remain at substantially the same value throughout operation. Alternately, operation may be begun with water in the catholyte compartment and electrolysis continued, albeit inefficiently, until the appropriate caustic value is reached. Obviously, it is also possible to charge the catholyte chamber with any convenient caustic concentration greater or less than 36 percent and allow the system to reach the desired equilibrium.

As suggested above, once the desired optimum sodium hydroxide concentration is established, this value is maintained by controlling the concentration of sodium chloride in the anolyte within the stated range. lt will become apparent that this control may be achieved by any combination of brine flow rates and concentrations, considered together with the degree of depletion occurring within the anolyte chamber. Thus it is possible to use a high feed rate of a brine having a concentration approaching that desired in the anolyte chamber. On the other hand, a feed brine approaching saturation may be employed, if introduced to the cell at a sufficiently slow rate.

In order that those skilled in the art may more readily understand the present invention and certain preferred embodiments by which it may be carried into effect, the following specific example is afforded.

EXAMPLE The cell comprises a cathode compartment containing a steel mesh electrode and separated from an anode compartment containing an expanded titanium elect d bearing 2T Qz=R 2 m l rat o pst ns 9. t surface, by a duPont XR cation-exchange membrane as described above and having a thickness of 20 mils, a gel water content of 25 percent and an equivalent weight of 1,150. The catholyte compartment initially contains a 36 percent solution of caustic. A sodium chloride solution, having a pH of 3.0, is fed to the anolyte compartment wherein electrolysis is conducted at an applied anode current density of l a.s.i. and a cell temperature of about C.

In the initial production run, the feed brine contains 303 grams per liter of sodium chloride and is introduced to the anolyte compartment at a rate of about 358 milliliters per minute, giving an average anolyte concentration of 287 grams per liter of sodium chloride. Under these conditions anolyte overflows at a rate of about 340 mls. per minute while the flow from the catholyte compartment is on the order of 3.4 mls. per minute. The sodium hydroxide concentration in the catholyte rises rapidly, at a rate of about 4 percent per day, the majority of the run being within the range of 41-54 percent sodium hydroxide. The average current efficiency for this period of operation is 40.4 percent.

The second production run is identical, with the exception that a brine having a concentration of 160 grams per liter sodium chloride is fed to the anolyte compartment at the rate of about 300 milliliters per minute, resulting in an average anolyte concentration of 134 grams per liter sodium chloride. Under these conditions an anolyte overflow of 276 mls. per minute and a catholyte flow rate of about 12 mls. per minute are measured. The product is a constant 36 percent sodium hydroxide solution with the cell operating at a current efficiency of 79.8 percent.

In another run like results are obtained with a similar membrane having a thickness of 20 mils and containing 25 percent gel water, an average anolyte concentration of 150 grams per liter NaCl and a 36 percent caustic solution. With a 4 mil membrane having a woven teflon backing and containing 25 percent gel water, an average anolyte concentration of 220,grams per liter NaCl is followed by a 37 percent caustic catholyte.

While the invention has been described with reference to certain preferred embodiments thereof, it is not to be so limited since changes and alterations may be made therein which are well within the intended scope of the appended claims.

We claim:

1. In a process for the electrolysis of an aqueous sodium chloride solution in an electrolytic cell the anolyte and catholyte compartments of which are separated by an electrolytically-conductive, hydraulically impervious, cation permselective membrane, the improvement which consists essentially of establishing an initial sodium hydroxide concentration in the catholyte within the range of 31-43 percent, and maintaining said sodium hydroxide concentration within said range by controlling the average sodium chloride concentration of the anolyte within the range of -250 grams per liter, the sole source of water to the catholyte being that transported through said membrane.

2. In a process for the electrolysis of an aqueous sodium chloride solution in an electrolytic cell the anolyte and catholyte compartments of which are separated by an electrolytically-conductive, hyraulically imprevious, cation permselective membrane, the improvement which consists essentially of establishing an initial sodium hydroxide concentration in the catholyte of about 36-38 percent, and maintaining said sodium hydroxide concentration within said range by controlling the average sodium chloride concentration of the anolyte within the range of 120-250 grams per liter, the sole source of water to the catholyte being that transported through said membrane. 

2. In a process for the electrolysis of an aqueous sodium chloride solution in an electrolytic cell the anolyte and catholyte compartments of which are separated by an electrolytically-conductive, hyraulically imprevious, cation permselective membrane, the improvement which consists essentially of establishing an initial sodium hydroxide concentration in the catholyte of about 36-38 percent, and maintaining said sodium hydroxide concentration within said range by controlling the average sodium chloride concentration of the anolyte within the range of 120-250 grams per liter, the sole source of water to the catholyte being that transported through said membrane. 